STOCKHOLM (AFP) - WikiLeaks aims to release documents revealing that Swedish Foreign Minister Carl Bildt is a United States (US) spy as part of a 'smear campaign' to stop Sweden from extraditing founder Julian Assange to the United States, a Swedish daily reported on Wednesday. The whistle-blower website has threatened in an internal memo to publish a so far unknown diplomatic cable 'where Foreign Minister Carl Bildt is shown to have been an informant for the United States since the 1970s', the Expressen tabloid reported, saying it had seen the WikiLeaks memo. Mr Bildt 'will have to step down. This will be the end of his political career', an unnamed person with access to the unpublished diplomatic cable was quoted as saying. Mr Bildt himself reacted to the report on his official blog on Wednesday challenging WikiLeaks to publish 'this in their opinion damning report'.